The present invention relates to a method of predicting the formation of heat streaks on a metal strip such as a steel strip being cold rolled, and a thermal sensor roll to be used for the prediction method.
During cold rolling a metal strip such as a steel strip, the metal strip is likely burnt onto the surface of a work roll. The burning causes the formation of surface defects, i.e. so-called heat streak, on the surafce of the metal strip to be rolled. The surface of the work roll is also damaged by the burning. The damaged surface pattern of the work roll is transcribed onto the surface part of a succeeding metal strip.
The formation of heat streaks is substantially derived from burning caused by the destruction of oil film. For instance, in case where a metal strip is cold rolled with a high ratio of reduction or at a high rolling speed under the condition that luburication by oil film is poor or that a coolant provides too small of a cooling function, the temperature of the metal strip rises due to heat caused by friction between the metal strip and the work roll, the plastic deformation of the metal strip, the shortage of the luburicant oil, etc. Consequently, the pressure and/or temperature of the oil film in a roll bite rises, resulting in the destruction of the oil film which promotes the burning between the work roll and the metal strip to be rolled.
The formation of heat streaks causes various kinds of bad influences, such as the deterioration of the surface property of a rolled product, the reduction of yield ratio and the lowering of work efficiency. In this regard, it is necessary to control the temperature of the metal strip being rolled, just below the roll bite during cold rolling, under a predetermined value.
However, there has not been proposed so far a proper sensor for detecting the temperature of a metal strip in order to prevent heat streaks from occuring. On the actual stage, rolling conditions such as reduction ratio and rolling speed are preset in a safety range referring to experimental prediction, to perform cold rolling without the formation of heat streaks. Consequently, rolling operation using the capacity of a rolling mill to the maximum extent is not done, resulting in the lowering of efficiency of production.
There is proposed a method of directly detecting the temperature of a metal strip being rolled using a infrared radiation thermometer disposed at the exit part of a rolling mill, by Japanese Patent Application Laid-Open 62-199209. In this method, the radiation thermometer using infrared beams as the measuring range of wave length is received in a cylindrical vessel, and a light shielding plate and an air supply nozzle are disposed at the opened side of the cylindrical vessel. Compressed air is injected through the air supply nozzle onto the surface of the metal strip, so as to purge the vapor of rolling oil from the surface of the metal strip. Thus, the temperature of the metal strip is detected in non-contact state.
However, the atmosphere where the temperature of the metal strip is detected by the infrared radiation thermometer is generally contaminated with the vapor of rolling oil and so on. The surface of the metal strip is covered with the film of the rolling oil. In this situation, the radiation ratio from the surface of the metal strip is widely varied according to the conditions. In addition, the radiation ratio is also changed in response to the kind of metal strip to be rolled and the degree of acid pickling. These conditions affect as disturbance factors on detected values and lowers the reliability of detection. Besides, it is impossible to preset the radiation ratio under a constant condition. On the other hand, it is difficult to obtain the change pattern of the radaition ratio corresponding to the actual temperature of the metal strip. As a result, the detected value of the temperature of the metal strip with high accuracy is not expected, so that the formation of heat streaks can not be predicted with high reliablity.